#include <bits/stdc++.h>
using namespace std;

#define ll long long 
#define lll __int128
#define INF 0x3f3f3f3f
#define LNF 0x3f3f3f3f3f3f3f3fll
#define MOD 998244353
#define pii pair<int, int>
#define ld long double
#define fi first
#define se second
#define all(a) a.begin(), a.end()

#define N 110

int dp[N][N][2];

int main() {
#ifdef TEST
    freopen("zz.in", "r+", stdin);
#endif
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    cout << setprecision(18);

    int x, y, p, q; cin >> x >> y >> p >> q;    
    int sx = 0, sy = max(0, y - x - q);
    y -= sy;

    int tx = 0, ty = 0;
    int d = INF, stp = 0;
    while (stp < 1e5) {
        d = min(p - sx - sy, max(0, x - (y - q)));
        sx += d, x -= d;
        if (p - sx >= x) sy = 0, sx += x, x = 0;
        d = min((p - sx - sy) / 2, min(x, y));
        sx += d, sy += d, x -= d, y -= d;
        ++ stp;

        if (tx == 0) ty += sy, sy = 0;
        tx += sx, sx = 0;
        d = min(sy, max(0, tx + q - ty));
        ty += d, sy -= d;
        if (tx) d = max(0, ty - tx - q), sy += d, ty -= d;
        if (x == 0) break;
        ++ stp;
    }

    cout << (stp == int(1e5) ? -1 : stp) << '\n';
}   

// 3 3
// 2 2